Electropolishing of zirconium and zirconium alloys



p 8 r E. R. BOWERMAN ETAL 2,851,406

ELECTROPOLISHING OF ZIRCONIUM AND ZIRCONIUM ALLOYS Filed Nov. 18, 1954 6 Sheets-Sheet 2 0 I00 A IO/ 90 AVA YA VAVAAVA 3o AVAYYY 7o ACETIC e V v WATER ACID 40 6O 5 AA 5 AAM w 60 AAAA A A AAAAAYA 4o AVAVAVAYAVAVAVAVAIAVAVAVA 7o Awmmflmuzgmyx HYDROCHLOR IC.

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ELECTROPOLISHING OF ZIRCONIUM AND ZIRCONIUM ALL OYS Filed Nov. 18. 1954 6 Sheets-Sheet 3 FIG. 3.

7o ZACETIC 4O 0 %WATER AC |D I VW AAA A A YVY AA A 30 v vvvv \A A AA wexexexexe AvAmmmVA AVAVAYAYA NWA YAYAVAVAVAY .00 AVAVAVAvAv mAAA flv HYDROCHLORIC ETIC ACID AN DRIDE INVENTOR. EDWARD a. uazsv-ne ED WW 2. WERMAN ATToialvay Sept. 9, 1958 E. R. BOWERMAN Em 2,851,406

ELECTROPOLISHING OF ZIRCONIUM AND ZIRCONIUM ALLOYS Filed Nov. 18. 1954 e Sheets-Sheet a FIG. 6.

90 WA AvAvyyg Ayy e lo mwvgxgyeygvl Vl iOO A Mi"?? O I00 '5 7o 60 50 4o 20 IO "o z HYDROCHLORIC "/o ACETIC ACID ANHYDRIDE INVENTOR.

A 71m M a'rroelvey United States Patent ELECTROPOLISHING OF ZIRCONIUM AND ZIRCONIUM ALLOYS Edwin R. Bowerman, Whitestone, and Edward B. Saubestre, Elmhurst, N. Y., assignors to Sylvania Electric Products Inc., a corporation of Massachusetts Application November 18, 1954, Serial vNo. 469,692 7 Claims. (Cl. 204-1405) The present invention relates to the electrolytic treatment of metals, and in particular to improved methods and baths for electropolishi-ng zirconium and various alloys containing zirconium as an essential ingredient, such as zirconium-uranium alloys. Specifically, the invention relates to acetic-hydrochloric electrolytes of the type described in copending application Serial No. 464,758, filed October 26, 1954, in the name of the inventors herein.

In recent years, the increased commercial application of zirconium, apart from the small amounts used in steel alloys, has created a need for methods of treating zirconium parts to obtain highly polished surfaces, and/or objects to rather critical dimensional tolerances. Mechanical polishing and chemical etching have been used for these purposes, and have been successful to the extent and with the limitations of such procedures. Still further, known perchloric-acetic electropolishing solutions have been used for electropolishing zirconium and many of its alloys, but such solutions exhibit many shortcomings. Although extensive work has been done in an attempt to control the possibility of explosive reactions with such acetic-perchloric electropolishing "baths, the fact that industrial accidents still occur, attests to the dilficulties of maintaining controls in actual installations.

Even if such industrial hazard can be effectively controlled, and workers adequately assured as to the safety of handling such perchlori'c-acetic acid baths, such baths exhibit a number of shortcomings which limit their general industrial application. The proportions ofthe bath are extremely critical from the standpoint of obtaining useful electropolishing action and avoiding industrial hazard. Accordingly, it is necessary to use such baths immediately upon preparation, and the life of such baths is rather short. Further, such critical formulation makes it diflicult to reproduce the same electropolishing action from object to object. Still further, the baths exhibit narrow Working ranges of anode current densities, which frequently requires ample runs to determine the effectiveness of electropolishing.

There are a number of general requirements which must be met to bring about widespread commercial utilization of electrolytes for the anodic working of metals, such as zirconium and its alloys. Among these requirements are that the electropolishing bath results in good surface leveling leaving clean and unfilmed surfaces at the end of treatment, have a wide operating range of anode current densities, be relatively insensitive to changes in temperature of the bath, be capable of operating at room temperature and over a range of temperature changes normally encountered in most installations, be made up from inexpensive and easily obtained commercial grade chemicals, exhibit good electrical conductivity, facilitate operation at high current densities to achieve rapid levelling, exhibit a relatively broad range of anode current densities, be relatively easy to make up, and have a comparatively long life. Preferably, the bath should ice have broad ranges for the essential ingredients and be rather n0n-critical to the presence of contamination, such as might occur incident to drag in or drag out of metal parts.

Accordingly, it is an object of the present invention to provide improved methods and solutions for electrochemically working objects of zirconium and its alloys which exhibit one or more of the aforesaid advantages.

In an article entitled The safe use of perchloric-acetic electropolishing baths, appearing in Metal Finishing, November 1949, and written by Dr. Pierre A. Jacquet, there is a detailed discussion of the use of perchloricacetic baths for the electropolishing of various metals. Consideration of these solutions and others suggested by Jacquet, indicate that the solutions are not well adapted for electro-Working zirconium and zirconium alloys. In particular, perchloric-acetic baths have a relatively narrow Working range of current densities, are extremely critical in formulation, are sensitive to temperature, require rigorous control over ambient temperature, and are formulated of rather expensive materials. Apart from these mentioned limitations, the precautions must be taken against the occurrence of electrical shorts to isolate plastics and other foreign or organic materials from the solutions, and to control evaporation such that the formulation of the solution is maintained within rather rigorous limits. Even with extreme precaution in handling, there is still present the risk of industrial accident and the attendant psychological discomfort to workers.

Accordingly, it is-a further object of the present invention to provide improved methods and solutions for electrochemically working zirconium and zirconium alloy objects which are safe in use, and are extremely noncritical with respect to formulation techniques of handling, and operating conditions.

It is a still further object of the present invention to provide electropolishing baths for zirconium and its alloys, which leaves treated surfaces bright, highly reflective and substantially free of films.

It is a still further object of the present invention to provide improved electropolishing baths for zirconium and its alloys which are capable of operating at comparatively high current density and within relatively broad ranges of current densities such as to achieve rapid and uniform polishing of the metal surfaces.

We have found that an electropolishing bath consisting essentially of an acetic-hydrochloric acid solution is highly suitable for anodic polishing of zirconium and its alloys, such as zirconium-uranium alloy. The presence of relatively large percentages by volume of hydrochloric acid in a hydrochloric-containing solution for electropolishing, is contrary to the teachings of the printed literature which indicates that best polishing action is obtained in the absence of large amounts of hydrochloric acid, and that the presence of small amounts of hydrochloric acid is detrimental to polishing action.

Electropolishing solutions in accordance with the present invention and characterized by the presence of relatively large amounts of hydrochloric acid in an acetictype bath are extremely desirable as compared to perchlori-c acetic type baths in that there is no risk of industrial explosion. Still further, the present acetic-hydrochloric solutions are not appreciably sensitive to water content, and are capable of achieving good polishing action in the presence of exceptionally large quantities of water.

The above brief description, as well as further objects, features and advantages of the present invention will be best appreciated by reference to the following detailed description of presently preferred electropolishing baths and treatment methods, when taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a ternary or triangular coordinate diagram illustrating electropolishing solutions coming within the scope of the present invention and suitable for electro-' chemical working of Zirconium; I I

Fig. 2 is a ternary diagram illustrating the low r operating limit of current densities for the zirconium electropolishing solutions of Fig. 1;

Fig. 3 is a ternary diagram illustrating electropolishing solutions according to the present invention for electrochemical working one Zirconium alloy;

Fig. 4 is a ternary diagram illustrating the lower operating limit of current densities for the electropolishing solutions of Fig. 3;

Fig. 5 is a ternary diagram illustrating electropolishing solutions according to the present invention suitable for electrochemically working a further zirconium alloy; and,

Fig. 6 is a ternary diagram illustrating the lower operating limit of current densities for the electropolishing solutions of Fig. 5.

Referring now specifically to Figs. 1 and 2 of the drawing, there are illustrated ternary diagrams for aqueous acetic-hydrochloric solutions suitable for electropolishing zirconium in accordance with the present invention. The three coordinates of the ternary diagram of Fig. 1, as well as in the remaining diagrams to be described hereinafter in detail, represent water which is present in increasing quantities along lines parallel to and progressively removed from the zero percent water axis 10; hydrochloric acid which is present in increasing quantities along lines parallel to and progressively removed from the zero percent hydrochloric acid axis 12; and acetic acid which is present in increasing quantities along lines parallel to and progressively removed from the zero percent acetic acid axis 14. The reading of such ternary diagrams is well known in the art, and accordingly further description will be dispensed with.

The acetic-hydrochloric acid solutions for electropolishing zirconium are found in the regions approximately enclosed by the solid lines in the triaxial diagram of Fig. 1, specifically by the solid straight line AB coinciding with the zero percent water axis 10, the irregularly curved line BCDE, and the solid straight line EA which coincides with the zero percent acetic acid axis.

Preferred compositions of zirconium-electropolishing solutions with which best polishing action is obtained lie within the lesser region bounded by the straight line FG coinciding with the zero percent water axis and the dashdash curved line GHF. Even when operating within the preferred region FGHF, it has been found that solutions having a water concentration of less than 19% yield a still more desirable polishing action, particularly with respect to pitting.

In the regions of the ternary diagram of Fig. 1 above and to the left of the line BCDE no polishing action is observed; in the regions intermediate the curved lines CHF and the curved line BCDE a semi-bright polishing action is observed; and in the region bounded by the lines FGHF bright polishing action is observed, although for water concentration above 19% in this most preferred region occasional pitting accompanies ing action.

The approximate coordinates of the points defining the overall and preferred zirconium-electropolishing regions in Fig. 1 are as follows:

A. water, 0% acetic acid, and 100% hydrochloric R ga water, 85% acetic acid, and 15% hydrochloric C. il water, 65% acetic acid, and 15% hydrochloric D. water, 25% acetic acid, and 40% hydrochloric acr the bright polish- E. 41% water, 0% acetic acid, and 59% hydrochloric acid;

F. 0% water, 14% acetic acid, and 86% hydrochloric acid;

G. 0% water, 67% acetic acid, and 33% hydrochloric acid;

H. 33% water, 22% acetic acid, and 45% hydrochloric acid.

Selection of a particular Zirconium electropolishing solution coming within the overall or preferred region depends upon practical considerations, such as the required width of the operating range and the power consumption. The wider the operating range, the more uniform will be the polishing action on recessed or irregularly shaped objects, while the lower the total amount of current required the smaller the required power source. It is to be expressly understood that no sharp transition occurs in the action of the bath of the electropolishing solutions as the outer limit of the defined electropolishing regions are approached. Rather the limits define threshhold regions wherein the polishing action may be other than optimum and/or where the power consumption becomes a prohibitive one.

Referring still further to Fig. 1, there is shown gradient lines 16, legended 1500 and 2000 which divide the overall zirconium-electropolishing region enclosed within the solid lines ABCDEA into three operating ranges of anode current density. Specifically, above and to the left of the gradient line 1500 the permissible operating range is less than 1500 amperes per square foot, between the gradient lines 1500 and 2000 the permissible operating range is between 1500 and 2000 amperes per square foot, and in the region bounded by the gradient line 2000, the permissible operating range is 2000 amperes per square foot.

Referring now specifically to Fig. 2, there is shown a ternary diagram illustrating the lower operating limit of current density (stated in amperes per square foot) for zirconium-electropolishing solutions in accordance with the present invention. By reference to Fig. 2, the lower operating limit of current density can be ascertained for a given solution; and by reference to Fig. 1 it is possible to ascertain the permissible operating range in amperes per square foot for the given solution. It is to be understood that the current density values for the lower operating limit in the respective regions are approximations and that said values may vary as adjacent regions are ap proached.

From the foregoing, the conjoint reading of the triaxial diagrams of Figs. 1 and 2 to ascertain the operating range and lower operating limit of current density for the selective zirconium-electropolishing solution should be understood. The following example is set forth for the purposes of illustration:

The point on the triaxial diagram of Fig. 1 represented by the letter P indicates an electropolishing solution consisting essentially of 15% water, 40% acetic acid, and 45 hydrochloric acid. For the solution 1?, which lies within the preferred electropolishing region FGHF and at a Water .concentration of less than 19% the permissible operating range of anode current density is between 1500 and 2000 amperes per square foot. Upon reference to the corresponding point P in Fig. 2 it will be seen that the lower operating limit is between 500 to 750 amperes per foot. In that the point P lies about in the middle of the 500 to 750 region of Fig. 2, it will be appreciated that the lower operating limit should be selected from the middle of the range. Thus a range of operation for the points P, P might be selected between a lower value of approximately 600 amperes per square foot and an upper value of between 2100 and 2600 amperes per square foot.

The following examples are illustrative formulations for electropolishing pure zirconium with acetic-hydra M vention, which formulations are listed in percentages by volume:

Table I Acetic Acid Hydrochloric Acid this 001mm mu' usaawoucam MOOOUIQU'IO HROCAIM OOOOOOQQ Referring now specifically to Fig. 3, there is shown the ternary diagram for acetic-hydrochloric acid solution suitable for electropolishing a zirconium alloy .consisting essentially of 80% of zirconium and 20% uranium. The ternary diagram of Fig. 3 further illustrates electropolishing'solutions which consist essentially of acetic anhydride and hydrochloric acid for electropolishing the 80% zirconium-20% uranium alloy in accordance with the present invention. For the sake of convenience in illustration, the acetic anhydride-hydrochloric acid solutions are illustrated along a secondary zero percent Water axis offset from the main axis 10, along which secondary axis increasing quantities of hydrochloric acid are read from left to right using the numbers on the axis 10 and increasing quantities of acetic anhydride are read from right to left using the numbers on the axis 10. r

The acetic-hydrochloric acid solutions suitable for electropolishing zirconium-uranium alloy are found in the regions approximately enclosed by the solid line in the triaxial diagram of Fig. 3, specifically by the solid straight line I] coinciding with the zero percent water axis, the solid curved line JKL and the solid straight line LI coinciding with the zero percent acetic acid axis.

Preferred compositions for the alloy-electropolishing solution of Fig. 3 with which best polishing action is obtained lie within the lesser region bounded by the solid straight line MN coinciding with the zero percent water axis, and the curved dash-dash line NOQM.

The acetic anhydride-hydrochloric acid solutions suitable for electropolishing the 80% zirconium-20% uranium alloy are found along the offset zero percent water axis 10 and along the solid line MN'J.

The approximate coordinates of the points defining the electropolishing regions for the 80% zirconium-20% uranium alloy in Fig. 3 are as follows:

M. 96% hydrochloric acid, and 4% acetic anhydride; N. 41% hydrochloric acid, and 59% acetic anhydride; J. 27 hydrochloric acid, and 73% acetic anhydride.

Further referring to Fig. 3, there is shown gradient lines 18, legend 2500 and 5000 which divide the overall alloy electropolishing region enclosed in the solid lines IJKLI into three operating regions of anode current densities. Specifically, above and to the left of gradient line 5000, the permissible operating range of anode .current densities is greater than 5000 amperes per square foot; between the gradient line 5000 and 2500 the permissible operating range is between 2500 and 5000 amperes per square foot; and within the region bounded y the gradient line 2500 the permissible operating range is from 1000 to 2500 amperes per square foot. Likewise reading along the offset zero percent water axis 10 the acetic anhydride-hydrchloric acid solutions have similar operating ranges of anode current densities.

Referring now specifically to Fig. 4, there'is shown a ternary diagram illustrating the low operating limit of current density (stated in amperes per square foot) for the electropolishing solutions of Fig. 3.

closed by the various gradients are approximations and the values may vary as the limit of any given region is approached.

In that the conjoint reading of the triaxial diagrams of Figs. 3 and 4 ascertain the operating range or operating limit of current. density for the selected electropolishing solution is the same as that described in Figs. 1 and 2, an illustrative example is dispensed with in the interests of brevity.

The following are illustrative formulations for electropolishing the zirconium-20% uranium alloy with acetic acid-hydrochloric-acid solutions in accordance with the present invention, which illustrative formulations are illustrated in percentages by volume.

Table II Acetic Hydro- Sclution Acid chloric Water Acid The following are illustrative formulations consisting essentially of acetic anhydride and hydrochloric acid for electropolishing the 80% zirconium-20% uranium alloy, which formulations are listed in percentages by volume.

Table III Solution Acetic Hydro- Anhydride chloric Acid Referring now specifically to Fig. 5, there is shown a I electropolishing zircaloy in accordance with the present invention.

The acetic hydrochloric acid solutions suitable for electropolishing zircaloy are found in the regions approximately enclosed by the solid lines in the triaxial diagram of Fig. 5, specifically by the solid line RS coinciding substantially with the zero percent water axis, the curved line STU and the straight line UR coinciding substantially with the zero percent acetic acid axis.

Preferred compositions for electropolishing zircaloy lie within the lesser and preferred region bounded by the straight line RV coinciding substantially with the zero percent water axis, the curved dash-dash line VWX, and the straight line XR coinciding substantially with the zero percent acetic acid axis.

The acetic anhydride-hydrochloric acid solutions suitable for electropolishing zircaloy are found along the offset zero percent water axis 10 and along the solid lines RY.

In the regions of the ternary diagram of Fig. above and to the left of the curved line STU, no polishing action occurs; in the regions between the curved line STU and the curved line VWX a semi-bright polishing action occurs; and in the regions bounded by the lines RVWXR a bright polishing action is observed.

The approximate coordinates of the points defining the overall and preferred electropolishing regions for zircaloy shown in Fig. 5 are as follows:

R. 0% water, 0% acetic acid, and 100% hydrochloric acid;

S. 0% water, 84% acetic acid, and 16% hydrochloric acid;

T. 64% water, 18% acetic acid, and 18% acid;

U. 55% water, 0% acetic acid, and 45% hydrochloric acid;

V. 0% water, 82% acetic acid, and 18% hydrochloric acid;

W. 32% water, 45% acetic acid, and 23% hydrochloric acid;

X. 43% water, 0% acetic acid, and 57% hydrochloric acid;

R. 100% hydrochloric acid, and 0% acetic anhydride;

Y. 23% hydrochloric acid, and 77% acetic anhydride.

Referring still further to Fig. 5, there is shown gradient lines 20, legend respectively 2500 and 5000 which divide the overall electropolishing region for zircaloy enclosed within the solid lines into three operating ranges of anode current densities. Specifically, between the curved line STU and the gradient line 2500, the operating range is less than 2500 amperes per square foot; within the region between the gradient line 2500 and the gradient line 5000 the operating range is between 2500 and 5000 amperes per square foot; and in the region bounded by the gradient line 5000 and the solid lines coinciding with the zero percent water and acetic acid axes, the permissible operating range is in the excess of 5000 amperes per square foot.

Similarly, reading along the offset zero percent water axis between the points RY, ranges of anode current density may be determined for the various acetic anhydride-hydrochloric acid solutions.

Reference will now be made to Fig. 6, wherein there is shown a ternary diagram illustrating the lower operating limit of anode current densities for the electropolishing solutions shown in Fig. 5. As before, by reference to Fig. 6 of the drawings, the lower operating limit of current densities can be ascertained for a given solution, and by reference to Fig. 5, it is possible ascertain the permissible operating range for the given solution.

The following examples are illustrative formulations for electropolishing the ductile zirconium alloy zircaloy, with acetic acid-hydrochloric acid solutions in accordhydrochloric ance with the present invention, the illustrative formulations being listed in percentages by volume:

Table IV Acetic Hydro- Solution Acid chloric Water Acid The following examples are illustrative formulations consisting essentially of acetic anhydride and hydrochloric acid for electropolishing zircaloy in accordance with the present invention, the illustrative formulations being listed in percentages by volume.

Table V Acetic Hydro- Solution Acid chloric Water Acid The accompanying ternary diagrams serve to illustrate the relative proportions of acetic-hydrochloric solutions which are suitable for electropolishing zirconium and its alloys. However, the solutions may include other ingredients, such as other acids, metallic salts and contaminations due to normal operations. For example, in making up an electrolyte suitable for the anodic polishing of zirconium, the bath corresponding to the point P might be selected in that it lies within one of the preferred areas of the zirconiurn-electropolishing region. During the continuous use of such baths in the electropolishing of zirconium, or for that matter, either of the illustrative alloys, and others having similar characteristics and properties, the composition of the bath may change. This changing composition may be due to the anodic dissolution of zirconium into the bath and the possibility of loss or addition of water content due to evaporation, absorption, drag-in and drag-out. Despite such changes in composition as may occur during use, if the relative percentages of the essential ingredients expressed in percentage by volume remains in the preferred or less preferred regions for the respective materials being polished, the bath will continue to operate satisfactorily.

Upon a comparison of Figs. preciated that there are large coextensive or common areas through the several electropolishing regions for zirconium and its alloys. For example, the preferred region bounded by the'line FGHF in Fig. l is entirely confined within the electropolishing regions for the 80% zirconium-20% uranium composition of Fig. 3 and the zircaloy composition of Fig. 5. It will be thus appreciated that solutions lying within this preferred region are not only suitable for the electropolishing of pure zirconium but also for the zirconium-uranium alloy, the ductile alloy known as zircaloy, and still further alloys of more or less the same essential composition.

in the illustrative embodiments and throughout the specification, commercial grade hydrochloric acid having a specific gravity of 1.18 is employed. The several references to acetic acid are to glacial acetic anhydride (99.5%).

Numerous modifications and substitutions in the present process and baths will occur to those skilled in the art 1, 3 and 5, it will be apand accordingly the appended claims should be given a latitude of interpretation consistent with the present disclosure; at times certain features of the invention will be used without a corresponding use of other features.

What we claim is:

1. The method of electropolishing a zirconium part including the steps of making said part anode in an electrolytic solution consisting essentially of hydrochloric acid acid and acetic acid which are present in relative percentages by volume lying within the area defined approximately in the ternary diagram of Fig. l by the lines ABCDEA, and passing electric current through said anode in an amount suficient to obtain electropolishing action of said surfaces.

2. The method of anodically polishing an article having zirconium surfaces including the steps of making said article the anode in an electrolytic solution containing as essential ingredients Water, hydrochloric acid and acetic acid which are present in relative percentages by volume lying within the area defined approximately in the ternary diagram of Fig. 1 by the lines FGHF, and passing electrolytic current through said anode in an amount suflicient to obtain an electropolishing action of said zirconium surfaces as determined by Fig. 2 of the accompanying drawings.

3. The method of anodically polishing a zirconiumuranium alloy, including the steps of making the article the anode in an electrolytic solution containing as essential ingredients hydrochloric acid and acetic acid, said percentages being by volume in the relative percentages of said ingredients lying within the area defined in Fig. 3 of the accompanying drawing by the solid line TJKLT, and passing electrolytic current through said anode in an amount sufficient to obtain electropolishing action of said a-lloy surfaces as determined by Fig. 4 of the accompanying drawings.

4. The method of anodically polishing an article of a zirconium alloy containing small percentages of tin, iron, chromium and nickel including the steps of making said article the anode in an electrolytic solution containing as essential ingredients hydrochloric acid and acetic acid, said percentages being by volume in the relative percentages of said ingredients lying within the area defined in Fig. 5 of the accompanying drawing by the lines RSTUR, and passing an electrolytic current through said anode in an amount suflicient to obtain an electropolishing action of said alloy surfaces as determined by Fig. 6 of the accompanying drawings.

5. The method of anodically polishing an article having zirconium surfaces including the steps of making said article the anode in an electrolytic solution containing as essential ingredients Water, hydrochloric acid and acetic acid, the percentage by volume of water being less than 33%, the percentage by volume of hydrochloric acid lying Within the range of 24% to 86%, and the percentage by volume of acetic acid lying within the range of 7% to 67%, and passing electric current through said anode in an amount sufficient to obtain an electropolishing action of said zirconium surfaces.

6. The method of anodically polishing an article having zirconium surfaces including the steps of making said article the anode in an electrolytic solution containing as essential ingredients hydrochloric acid and acetic anhydride, the percentage by volume of hydrochloric acid lying within the range of 26% to 96%, and the percentage by volume of acetic anhydride lying within the range of 4% to 74%, and passing el ctric current through said anode in an amount sufiicient to obtain an electropolishing action of said zirconium surfaces.

7. The method of anodically polishing an article of an zirconium-20% uranium alloy including the steps of making said article the anode in an electrolytic solution containing as essential ingredients hydrochloric acid and acetic anhydride, the percentage by volume of hydrochloric acid lying within the range of 27% to 96%, and the percentage by volume of acetic anhydride lying within the range of 4% to 73%, and passing electric current through said anode in an amount sufficient to obtain an electropolishing action of said article.

References Cited in the file of this patent UNITED STATES PATENTS 2,057,272 Schumpelt Oct. 13, 1936 2,421,316 Carson et a1. May 27, 1947 2,463,190 Lundbye Mar. 1, 1949 FOREIGN PATENTS 530,041 Great Britain Dec. 4, 1940 

1. THE METHOD OF ELECTROPOLISHING A ZICONIUM PART INCLUDING THE STEPS OF MAKING SAID PART ANODE IN AN ELECTROLYTIC SOLUTION CONSISTING ESSENTIALLY OF HYDROCHLORIC ACID ACID AND ACETIC ACID WHICH ARE PRESENT IN RELATIVE PERCENTAGES BY VOLUME LYING WITHIN THE AREA DEFINED APPROXIMATELY IN THE TERNARY DIAGRAM OF FIG. 1 BY THE LINES ABCDEA, AND PASSING ELECTRIC CURRENT THROUGH SAID ANODE IN AN AMOUNT SUFFICIENT OT OBTAIN ELECTROPLOLISHING ACTION OF SAID SURFACES. 